(1) Field of the Invention
The present invention relates to an optical transmission system and a signal speed converting apparatus, and more specifically to an optical information transmission system for transmitting and receiving communication frames via an optical fiber transmission line and a signal speed converting apparatus to be used therefore.
(2) Description of the Related Art
In information devices, such as a large-capacity router, a storage server, a high speed optical line concentrator, or the like, or a communication node device, an optical interface speed for transmitting information included in each device is rapidly increasing from 1 Gbps to 10 Gbps and from 10 Gbps to 40 Gbps with the increase of information load to be treated. The reason is that using a high speed optical interface can reduce a mounting volume of the apparatuses or power consumption thereof as well as the number of optical lines connecting between the apparatuses to save installation costs of the lines or rental costs thereof, as compared to using a plurality of low speed optical interfaces.
If a range between the information devices is short, it is able to interconnect the apparatuses through an optical interface for short-range transmission wherein maximum transmission range is about 10 km. However, for example, when interconnecting the information devices distributed within a city (within a range of about 100 km) or between cities (about 100 km or more), they should pass through a wavelength division multiplexing (WDM) optical transmission network, such as a metropolitan network or a trunk line network, etc.
These kinds of information devices are provided with short-range optical interfaces, such as, for example, SONET/SDH, OTN, Ethernet LAN-PHY/WAN-PHY, etc., as connection interfaces to the WDM optical transmission network. A transmitting-side information device uses one of the interfaces to transmit an information signal as an optical signal in a short-range transmission format to the WDM optical transmission network. The optical signal transmitted from the information device is converted into an optical signal (for example, an optical signal of 1.5 μm band) for long-range transmission having a predetermined speed by WDM transmission equipment (for example, add drop multiplexer (ADM)) belonging to a communication carrier. The optical signal thus converted is wavelength-multiplexed with optical signals transmitted from other information devices and transmitted over an long-range optical section. The WDM optical signal transmitted over the long-range optical section is converted back into the optical signal in the short-range transmission format by receiving-side WDM transmission equipment and transmitted to an opposite-side information device.
Until now, a modulation speed of the optical interface for long-range transmission is also being increased to 2.5 Gbps, 10 Gbps, and 40 Gbps as the speed of the optical interface for short-range transmission is increased. In the WDM optical transmission network section, however, if the line speed is increased, the wavelength dispersion within the optical path is affected so that it becomes difficult to perform the long-range transmission of the optical signal.
To be concrete, if the modulation speed of the optical signal is increased, waveform distortion is increased by the wavelength dispersion so that the signal transmission range is reduced in inverse proportion to a square of bit rate. For this reason, the maximum transmission range is reduced with the increase of the transmission speed. For example, the maximum transmission range is 600 to 1200 km in 2.5 Gbps, it is reduced to several tens km in 10 Gbps, and it is reduced only to several km in 40 Gbps. Therefore, if the modulation speed of the optical interface for long-range transmission becomes high, the number of relay devices required between WDM transmission end offices is increased, thereby greatly raising costs.
A mismatch between the transmission speed of the optical interface for short-range transmission in each information device and the modulation speed of the WDM optical transmission network forming the long-range transmission section may also cause serious problems afterward. Since the transmission speed of the interface for short-range transmission, such as, for example, 10 Giga Ethernet, 40 Giga Ethernet, 100 Giga Ethernet, etc., can relatively easily be increased, it can be expected to rapidly progress the development of the high speed interface.
However, since the WDM transmission section has problems in the reliability of the optical line or in the increased cost due to the restriction of the aforementioned transmission range, the modulation speed of the interface for long-range transmission cannot be changed simply. For this reason, the optical line of 10 Gbps to 2.5 Gbps is mainly used up to now and the introduction of the high speed line of 40 Gbps is rare worldwide. Even if the 100 Giga Ethernet is adopted as a next-generation interface for short range transmission in future, when the channel speed of the WDM optical transmission equipment is 40 Gbps or less, there may be a risk of hindering the signal transmission among the long-range information devices within a city or between cities.
As related arts concerning the above-mentioned problems, for example, a WDM optical communication system using a wavelength demultiplexing optical transponder is disclosed in Japanese Patent publication No. 2001-230759 and WO/1998/042095.
FIG. 7 shows an example of an optical network in the related art using the wavelength demultiplexing optical transponder. In FIG. 7, a signal transmission system from a first office 182-1 to a second office 182-2 is shown.
The first office 182-1 includes a transmitting-side WDM transmission end office 183 to which routers 150-1 to 150-3 are connected via intra-office optical lines 187-1 to 187-3. The second office 182-2 includes a receiving-side WDM transmission end office 184 to which routers 150-4 to 150-6 are connected via intra-office optical lines 187-4 to 187-6.
The transmitting-side WDM transmission end office 183 comprises a wavelength demultiplexing transponder 180, transponders 185-1 and 185-2, and an optical wavelength multiplexer 146 for wavelength-multiplexing optical signals having different wavelengths (wavelengths λ1 to λ6) output from these transponders and transmitting them to a trunk line WDM optical path 142. On the other hand, the receiving-side WDM transmission end office 184 comprises an optical wavelength demultiplexer 144 for demultiplexing WDM optical signals received from the trunk line WDM optical path 142 into optical signals per wavelength, a wavelength demultiplexing transponder 181 connected to the optical wavelength demultiplexer 144, and transponders 186-1 and 186-2.
The router 150-1 sends out transmission signals modulated, for example, at a high speed of 40 Gbps or 100 Gbps to the intra-office optical line 187-1. The transmission signals are subjected to a wavelength conversion and a format conversion into inter-office transmission signals by the wavelength demultiplexing transponder 180 and thereafter demultiplexed into low-speed optical signals with four wavelengths λ1 to λ4, thereby to input to the optical wavelength multiplexer 146. Transmission signals from the router 150-2 and 150-3 are input to the transponders 185-1 and 185-2 via the intra-office optical lines 187-2 and 187-3 and input to the optical wavelength multiplexer 146 as the low-speed optical signals with wavelengths λ5 to λ6, respectively. The optical wavelength multiplexer 146 wavelength-multiplexes the optical signals with the wavelengths λ1 to λ6 and send outs the wavelength-multiplexed signals to the trunk line WDM optical path 142.
In the receiving-side WDM transmission end office 184, the WDM optical signals received from the trunk line WDM optical path 142 are demultiplexed into optical signals per wavelength by the optical wavelength demultiplexer 144. The four optical signal trains with the wavelengths λ1 to λ4 are input to the wavelength demultiplexing transponder 181 to absorb an inter-wavelength delay difference occurred on the optical path 142 and to convert into an original multiplexed optical signal. The multiplexed optical signal is output to the router 150-4 via the intra-office optical line 187-4. The optical signal with λ5 and the optical signal with λ6 output from the optical wavelength demultiplexer 144 are wavelength-converted by the transponders 186-1 and 186-2 and input to the routers 150-5 and 150-6 via the intra-office optical lines 187-5 and 187-6, respectively.
Although FIG. 7 shows the information transmission system from the first office 182-1 to the second office 182-2, each of the offices 182-1 and 182-2 generally includes both the transmitting-side WDM transmission end office 183 and the receiving-side WDM transmission end office 184 so as to enable the information transmission from the second office 182-2 to the first office 182-1.
FIG. 8 shows a configuration view of the wavelength demultiplexing optical transponder in the related art. In FIG. 8, one wavelength demultiplexing optical transponder 196 is constituted by a combination of the transmitting-side wavelength demultiplexing transponder 180 and the receiving-side wavelength demultiplexing transponder 181 shown in FIG. 7.
The optical signal (wavelength λa=1.3 μm, etc.) input from the intra-office optical fiber 187-1 is converted into an electrical signal by an optical receiver 191 and demultiplexed into N (four in FIG. 8) low-speed transmission signals by a demultiplexer 193. These transmission signals are generally transmitted as parallel signals in an electric circuit. The number N of low-speed signals in the specification means the number of logical signal lines.
The N demultiplexed transmission signals are input to optical transmitters 194-1 to 194-4 respectively, converted into optical signals each having a format for inter-office transmission, and transmitted from the optical transmitters as optical signals having different wavelengths (λ1 to λ4) to output optical fibers 188-1 to 188-4 connected to the optical multiplexer.
The optical signals having the wavelengths λ1 to λ4 received from optical fibers 189-1 to 189-4 connected to the receiving-side wavelength demultiplexing transponder 181 are input to optical receivers 195-1 to 195-4 respectively to convert into electrical signals and compensate for the inter-signal delay difference occurred in the trunk line optical path. The output signals from the optical receivers 195-1 to 195-4 are multiplexed by a multiplexer 192 and input as an electrical signal train to the optical transmitter 190. The optical transmitter 190 converts the input electrical signal train into a high-speed optical signal with a wavelength λb and transmits the high-speed optical signal to the intra-office optical fiber 187-4.
In order to solve the aforementioned problems, the wavelength demultiplexing transponder according to the prior art time-demultiplexes (interleaves) the high-speed optical signals to divide them into a plurality of low-speed optical signals with different wavelengths from each other so that wavelength-multiplexed signals are transmitted from the inter-office side optical transmitter, like the operation of the aforementioned transmitting-side wavelength demultiplexing transponder 180.